"I broke my penis once trying to backflip a motorcycle. That was ******." Ladies and gentlemen, allow us to introduce Johnny Knoxville: a man described by Wikipedia as 'an American daredevil', which is about as big an understatement as saying 'cheese: it's a bit fattening'.

But this isn't really news, because Knoxville made his name injuring significant parts of his anatomy for the entertainment of millions on Jackass; a series with such inherent perversity it's basically a middle-fingered salute to the iron-willed Health and Safety diktat. Very Top Gear, then...

As such, Johnny's always hurting. "I get a lot of breaks and sprains and concussions," he casually muses. When TG meets him in a posh central London hotel - in town promoting new Jackass flick Bad Grandpa - he's sporting a cast over his left arm. "Right now I have a fractured elbow, and I, um, just ruptured my finger."

For him, it's all in a day's work, in this case while filming for Bad Grandpa; in it, Knoxville gets made up to be an 86-year-old who has to transport his grandson to his estranged son with the help of a mint-green 1970s Lincoln Continental coupe.

"I liked those old 70s looking cars," he says, "but that big boxy shape [of the Lincoln], that's what did it. We looked at a number of cars but in that colour it just looked like an old man's car. We had two of them, but neither one of them ******* ran."

You might have noticed that Knoxville is a little bit sweary, but the sentiment echoes a theme running through his motoring history: Stuff Tends Not To Work.

"My first car was a 1965, fire engine red Mustang. It was a great car." He pauses, looks at me, and then laughs. "Actually it was a piece of **** but it was a great first car. The seat was always broke, so it looked like I was staring at the ceiling. It had a three speed 289 engine [that's a 4.7-litre, small-block 'Windsor' V8, fact fans] and broke down constantly, but as much as it gave me problems, I loved that car. It had character.

"I loved the old Mustang," he says, "but my Eldorado, I really love." He's talking about his current car, a 1970 black Cadillac (he's got a new one too, but on this he admits "I don't even know which one it is man, that's how much I pay attention"). No, his 1970 Eldorado - which we told you about earlier this year when we caught up with Schwarzenegger and Knoxville for The Last Stand - has a lot of history tied into it. "My daughter was tiny when I got it and now she's 17. It's got a lot of memories."

A beat. "That ******* thing man, at one point I had two new engines in it over a four year span, and after filming Jackass 3D I had everything fixed on it. For a while I just wanted to keep it looking bad on the outside because it had so much character."

He changed his mind and kept it looking factory fresh, though he can't say the same about an old XJ6 he and his Jackass co-stars Chris Pontius and Steve-O acquired for the Gumball 3000 in 2001. "We bought this beat up 1989 Jaguar [pronounced Jag-wire] from a chop shop at the last second. It was a literally beaten up Jag." Unfortunately, the remainder of this anecdote is unsuitable for TopGear.com; suffice to say, Pontius and Steve-O made the journey from London so uncomfortable, Johnny switched cars as soon as they landed in France "into a mom and pop type minivan".

But then, that's been the Jackass MO. "When you're doing a prank movie failure is just a huge part of it," he tells me, reciting what could be a Top Gear motto. "You don't know what you're gonna get. Most of the time you fail all day long until you get that one reaction."

Ah yes, reaction. We think even Newton himself would be stretched to apply his classical third law to the Jackass maxim of 'getting that one reaction'. In one tense scene, Johnny's character Irving Zisman 'accidentally' crashes into and topples a biker gang's wheels outside a pub. Seriously bad move. But that wasn't the scary part. It was the 'man inside' whose life Knoxville was more concerned about.

"I was terrified for Chuck," he says, with a straight face, "because we had him in the bar 45 minutes ahead of time being a ****, and when it kicked off, they were holding him back; I mean they nearly broke his arm at one point, and he was like 'I give, I give, I give'.

"We didn't have that part in the movie, but then one guy said to him 'you're looking at an angel of death right now'. I mean, they're not ******* around, that was a scene where there was real peril at stake."

It was an incredibly tense scene in an otherwise hilariously crude and very Jackass movie, but we want to know what kind of havoc would occur if TG and Jackass ever collaborated. Johnny laughs out loud. "I'm sure if the two got together, bad stuff would happen. You don't have to write that stuff ahead of time, because chaos would just prevail."

Who, then, would like to see that?